An automatic tire-filling apparatus is described in Austrian patent 408,867 B and the Austrian patent application A 1274/2001. The vehicle speed and the actual tire pressure are measured in the system and a setpoint of the tire pressure is calculated, deviations of the actual value from the setpoint value are determined and based upon those deviations, a filling or emptying valve can be operated to bring the actual tire pressure to the setpoint value.
In general, air is used for this purpose and a compressor is provided to draw the atmospheric air from the ambient atmosphere (i.e. the surroundings), to compress it and to force the air via the pneumatically-actuated valve into the respective tires when the tire pressure is above the setpoint, or is released from the tires through those valves into the surrounding atmosphere.
The critical operating state of such a system for automatic tire pressure control is the filling of the system since for any particular vehicle speed, a tire pressure which is too low may be unsafe. In practice, efforts to utilize a supply reservoir for the filling fluid have proved to be unsatisfactory since in automatic operation, such reservoir could be rapidly depleted and the safety of the vehicle called into question. The result is a problem for management of vehicle systems and could not readily be resolved. As a consequence, closed systems utilizing pressurized storage vessels as the source of the tire-filling fluid have not proved to be highly satisfactory in the past.